JPH04700B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] This invention relates to a method for preparing a highly dispersed supported platinum catalyst comprising platinum and a carrier.

[Prior art and its problems]

Conventionally, as a platinum catalyst used in an electrochemical cell, a method using platinum black prepared by a well-known method is known. However, when platinum black was used as a catalyst, the specific surface area of platinum was small, and satisfactory characteristics could not be obtained. Therefore, in order to obtain a platinum catalyst having a large specific surface area, a method has been adopted in which a porous fine powder having a relatively high surface area, such as carbon black, is used as a carrier and platinum fine particles are supported on the carrier. Conventional techniques for preparing this platinum-supported catalyst can be broadly classified into the following three types.

The first method (hereinafter referred to as gas phase reduction method) involves impregnating a carrier with a platinum salt such as an aqueous solution of chloroplatinic acid, removing water by drying, and then directly heating the carrier at a predetermined temperature in a hydrogen stream. Methods of hydrogen reduction are known. However, in this method, depending on the type of carrier, the platinum salt may not be adsorbed onto the carrier surface, but may simply be absorbed into the pores between the carriers or within the carrier. By doing so, the platinum salt aqueous solution is gradually concentrated within the pores. As a result, the crystallite diameter of platinum increased (corresponding to a decrease in the specific surface area of platinum), and a platinum catalyst with a high surface area as expected could not be obtained. Furthermore, it has been difficult to uniformly disperse platinum on the surface of the carrier by concentrating the platinum salt aqueous solution. Furthermore, another problem is that in order to directly reduce the dried platinum salt-impregnated powder in a hydrogen stream to obtain a supported platinum catalyst, it is necessary to raise the temperature to a high temperature of usually 100 DEG C. to 400 DEG C. Therefore, when the treatment is carried out under these conditions, platinum sintering (increase in the diameter of platinum particles) occurs, and the platinum specific surface area further decreases.

Next, as a second method (hereinafter referred to as pyrolysis method), as disclosed in Japanese Patent Application Laid-Open No. 50-56545,
Dinitrodiammine platinum (Pt) as a platinum salt
(NH ₃ ) ₂ (NO ₂ ) ₂ ) was dissolved in 30% nitric acid, brought into contact with carbon black, thoroughly blended and dried. There is a method of thermally decomposing salt to obtain a supported platinum catalyst. However, even in this case,
Since the platinum salt is absorbed into powder and dried, the platinum crystallite size increases due to concentration similar to the gas phase reduction method, or the platinum dispersion state becomes non-uniform. Furthermore, during thermal decomposition of platinum salts, sintering of platinum and disappearance (oxidation) of carbon black, which is a carrier, are expected, and it is considered that a catalyst with the expected characteristics cannot necessarily be obtained.

Furthermore, as a third method (hereinafter referred to as liquid phase reduction method), as disclosed in JP-A-54-92588,
There is a method in which an aqueous solution of chloroplatinic acid is added to carbon black dispersed in water, the carbon black is brought into sufficient contact with the carbon black, and then the chloroplatinic acid is reduced using sodium dithionate. However, with this method, it is difficult to support a predetermined amount of platinum on the carrier.
It turned out to be impractical.

[Purpose of the invention]

This invention eliminates the drawbacks of the conventional method and can safely produce a uniformly dispersed platinum-supported catalyst in a short time without unnecessarily increasing the platinum crystallite diameter during the preparation stage of the platinum-supported catalyst. It is an object of the present invention to provide a method for reliably supporting a certain amount of platinum.

[Key points of the invention]

In this invention, in order to enable the platinum salt aqueous solution to contact the surface of the carrier well, the system uses a reducing agent to adjust the pH to an alkaline level after bringing the platinum salt into sufficient contact with the carrier that has been subjected to hydrophilic treatment. By reducing the platinum salt under conditions that allow sufficient reduction to occur, and at the same time adsorbing the reduced platinum onto the surface of the carrier, a high surface area, highly dispersed platinum supported catalyst with a small platinum crystallite diameter can be produced reliably and safely. Moreover, it can be prepared in a short time.

[Embodiments of the invention]

The present invention will be explained below based on examples.

Example 1 9g of acetylene black as a catalyst carrier
360 ml of a 10 wt % nitric acid aqueous solution was added, and the temperature was raised to 60° C. while stirring, and stirring was continued at this temperature for 3 hours. After 3 hours, it was filtered off and the cake was thoroughly washed with deionized water until pH=7. Next, this cake was mixed with a 2.1g-Pt/chloroplatinic acid aqueous solution.
The mixture was thoroughly dispersed in 500 ml and further stirred at room temperature for 2 hours so that the acetylene black and chloroplatinic acid were brought into sufficient contact with each other. A 0.1M aqueous sodium carbonate solution was added to this to adjust the pH to 9.5. After this, 213 ml of 0.1M aqueous formic acid solution was added dropwise to the stirring liquid over about 10 minutes. After the addition was completed, the reaction mixture was raised to 50° C. while stirring. After further raising the temperature, heating and stirring were continued for 2 hours. Thereafter, the reaction product was filtered off, and the cake was washed with 60°C warm water. After washing, remove the cake.
The platinum-supported catalyst was obtained by heating and vacuum drying at 50°C for 10 hours. As a result of evaluating the physical properties of the obtained catalyst, the platinum specific surface area was 180 m ² /g-Pt, and the amount of platinum supported was 10.4%.

In Example 1, a 10wt% nitric acid aqueous solution was used for the hydrophilic treatment of the catalyst carrier, but the same result can be obtained by using a 1.6 mol/hydrochloric acid aqueous solution, 1N sulfuric acid, or 0.1N acetic acid instead of the nitric acid aqueous solution. There are various other embodiments that can be modified without departing from the gist of the present invention. Below, some modified examples of the first example that obtained results similar to those of the first example will be introduced.

1 In Example 1, the carrier was pretreated at 80°C.
Use a carrier that has been used for a long time.

2 In Example 1, to adjust the PH
Use 0.1N-NaOH aqueous solution.

3 In Example 1, the pH was adjusted using ammonia water.
Adjusted to =9.5.

4 In Example 1, 0.1N-oxalic acid aqueous solution was used instead of 0.1M-formic acid aqueous solution, and the temperature was 60°C.
Get a refund for 1 hour.

5 In Example 1, reduction is performed at 80°C for 1 hour using 0.1N formaldehyde.

6 Instead of gradually dispersing the pretreated carrier in the aqueous chloroplatinic acid solution in Example 1, the pretreated carrier was dispersed in about 500 ml of deionized water and then
A chloroplatinic acid aqueous solution of mg-Pt/ml is gradually added with sufficient stirring to bring the support and platinum salt into sufficient contact.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the carrier is subjected to hydrophilic treatment before being brought into contact with the platinum salt aqueous solution, and the reduction reaction is uniformly carried out in the liquid phase. The contact between the carrier and the platinum salt became better, and a uniformly dispersed supported platinum catalyst having a wider specific surface area of platinum was obtained. The specific surface area of platinum in the conventional method is approximately 100~
120 m ² /g-Pt, whereas according to the method of the present invention, the specific surface area is 180 m ² /g-Pt, which is 1.5
~1.8 times. In addition, since a liquid phase reduction method is adopted, unnecessary heat is not applied during reduction of platinum salt.
Improved platinum sintering. Furthermore, since platinum was adsorbed onto the surface of the carrier during reduction, the dispersibility of platinum and the strength of its support on the carrier were improved. Furthermore, it was possible to obtain the required platinum-supported catalyst without repeating a series of steps. Furthermore, as mentioned above, since the platinum specific surface area increases,
When such a catalyst is used in an electrochemical cell such as a phosphoric acid fuel cell, the cathode reaction on platinum is promoted in proportion to the increase in area, and when compared at the same current density, the output voltage of the cell becomes higher. This has the effect of improving the characteristics of the fuel cell.

Claims (1)

[Scope of Claims] 1. A catalyst carrier made of carbon particles such as carbon black or acetylene black is brought into contact with an acid aqueous solution for hydrophilic treatment, and then sufficiently brought into contact with an acid aqueous solution of a platinum salt to bring the pH of the system to an alkaline side. A method for preparing a platinum-supported catalyst for an electrochemical cell, which comprises adding a reducing agent to the system, and then heating the system to a temperature at which reduction of the platinum salt can occur while stirring the system. 2. In the method described in claim 1,
A method for preparing a supported platinum catalyst, characterized by using a strong acid such as nitric acid, hydrochloric acid, or sulfuric acid as an acid aqueous solution. 3. In the method described in claim 1,
A method for preparing a supported platinum catalyst, characterized in that the hydrophilic treatment temperature is from 40°C to 90°C. 4. In the method described in claim 1,
A method for preparing a supported platinum catalyst, characterized in that the pH is 9 or more. 5. In the method described in claim 1,
PH adjustment reagents are NaOH, KOH, Na ₂ CO ₃ , K ₂ CO ₃
Alternatively, a method for preparing a supported platinum catalyst, characterized by using aqueous ammonia. 6. In the method recited in claim 1,
A method for preparing a supported platinum catalyst, characterized in that the platinum salt is chloroplatinic acid. 7. In the method described in claim 1,
A method for preparing a supported platinum catalyst, characterized in that the reducing agent is a reagent having a substituent corresponding to an aldehyde group, such as formic acid, formalin, or sodium formate. 8. In the method described in claim 1,
A method for preparing a supported platinum catalyst, characterized in that the reducing agent is hydrazine, sodium borohydride or oxalic acid. 9. In the method recited in claim 1,
A method for preparing a supported platinum catalyst, characterized in that the temperature at which platinum salt can be reduced is from 40°C to 90°C.